This invention relates to the reduction in the sound level of gas turbine engined helicopters.
Efforts have been made to reduce the noise caused by helicopter main rotor blades, with the result that the noise generated by the blade drive engine can now form a significant proportion of the sound level of a gas turbine engined helicopter.
Document U.S. Pat. No. 4,421,455 discloses a gas turbine engine, in particular for an APU (Auxiliary Power Unit), in which coatings forming sound attenuators are integrated at certain locations of metallic casing walls defining an air supply passage for a compressor of the engine. The thickness of the coatings means a substantial required volume affecting the dimensioning of the engine. In addition, the engine, in particular the metallic casing, has to be designed originally to allow the housing of the sound attenuators.